Recently, performance of image sensors becomes better and better, and size of chips in the image sensors becomes smaller and smaller. As the pixel size decreases, cost decreases greatly, whereas some performance such as dynamic range, sensitivity, signal-to-noise ratio can be limited. In order to improve the dynamic range of the chip, a pixel cell comprising a large pixel and a small pixel is often used. Similarly, a pixel cell comprising a large FD (floating diffusion) pixel and a small FD pixel is also often used. For the image sensor using the pixel cell comprising the large and small pixels, the large pixel has a high sensitivity and can sense an object with a low illuminance, and the small pixel has a low sensitivity and is used to sense an object with a high illuminance. Thus, an image obtained by this kind of image sensor includes image information of the large and small pixels, and corresponds to a composite image of the images obtained under a high sensitivity (large pixel) condition and a low sensitivity (small pixel) condition, thus reflecting more information under the low illuminance and high illuminance. Therefore, the pixel cell comprising the large and small pixels can realize a wide dynamic range of the image sensor. However, in order to realize the same resolution as the conventional image sensor, the size of the chip in the above image sensor must be increased, thus reducing cost performance.